The invention relates to a halftone digital image processing device, in particular, to an improvement in the resolution of information, such as characters and line drawings, processed by a device in which a halftone representation is performed using an area gradation method.
When recording images based on the dot matrix technique, a conventional recording device can adjust a density level of each of dots in order of four grades at maximum. In a digital color reproduction machine by way of example, however, a gradation representation of 64 grades is generally required for each of basic recording colors such as yellow (Y), magenta (M), cyan (C) and black (BK).
In case of such a multi-level representation, it has been customary heretofore that a relatively large dot region comprising a plurality of dots (e.g., 8.times.8) is defined as a unit of gradation processing region, and both the number of recording dots and the number of non recording dots are adjusted for each dot region to represent a density level of each gradation processing region. The halftone representation method of this type is called as an area gradation method.
A gradation representation using the area gradation method is mainly divided into a density pattern method and a dither method. For the density pattern method, the average density in a given processing region (e.g., 8.times.8) is determined and the resulting density is compared with each of values of a threshold table which have been present for each pixel of the processing region. The result of this comparison produces binary data of "1" or "0" for each pixel.
For the dither method, input data of each pixel is directly compared with a corresponding value of the threshold table one to one, and the result of this comparison produces binary data of "1" or "0".
In case of a matrix table of 8.times.8 generally thresholds of 0, 1, 2, . . . 62 and 63 are arranged in 64 pixel positions, respectively. The sequence of arrangement of the thresholds, i.e., the type of patterns, is divided into a dot concentration pattern and a dot dispersion pattern. FIG. 4c shows a typical dot dispersion pattern which is generally called as a BAYER type. FIG. 4e shows a typical dot concentration pattern which is generally called as a spiral pattern. Description will now be made by assuming one example. FIG. 4a shows some original image corresponding to a pixel region of 8.times.8. In this figure, the hatched section has the density of 44 and the remaining section has the density of 14. In other words, the figure represents a section in which the density is abruptly changes across an inclined edge. FIG. 4b shows density data for each of pixels read from the original image of FIG. 4a.
FIG. 4d shows the result of processing the density data of FIG. 4b based on the dither method using the dot dispersion pattern of FIG. 4f shows the result of processing the same density data based on the dither method using the dot concentration pattern of FIG. 4e, and FIG. 4g shows the result of processing the same data based on the density pattern method using the dot concentration pattern of FIG. 4e. The hatched pixels indicate data of "1" (recording pixel), while the remaining pixels indicate data of "0" (non recording pixel).
Referring to those results of processing, it will be found that although no substantial error exists in any of the results in relation to the gradation, there hardly appears information indicating specific characteristics of the input data except for the average gradation in the matrix of 8.times.8. Information on the edge of the input image barely appears in the result of dither processing using the dot dispersion pattern (FIG. 4d).
Stated otherwise, when the area gradation processing is carried out in units of (n.times.n) matrix, the resolution reduces down to 1/n as compared with the case where the gradation processing is carried out in units of one dot. In case of such images as photographs, for example, the recording quality can obtain high evaluation even with low resolution, if the halftone, i.e., the density of each pixel is represented accurately. In case of line drawings or characters, however, the reduced resolution immediately leads to a reduction quality.
Generally, images including line drawings or characters give a monochrome representation and do not require a gradation representation in many instances. It has been, therefore, proposed to switch the image information processing for selection of either one of binary processing and gradation processing in accordance with the content of image to be handled. But, when handling multicolor images, for example, pixel information of characters or line drawings requires to be handling as halftone data in order to reproduce various different colors. Also, in monochrome recording, it is sometimes desired to represent the characters or line drawings in halftone such as gray.